BOSTON — With Dzhokhar Tsarnaev seated at the defense table no more than 15 feet away Thursday, the father of an 8-year-old boy killed in the Boston Marathon bombing described the moment when he looked down at his son’s pale, torn body and realized that he wouldn’t make it.

“I saw a little boy who had his body severely damaged by an explosion,” Bill Richard told the jury, “and I just knew from what I saw that there was no chance, the color of his skin, and so on.”

Martin Richard was one of three people killed in the bombing near the race’s finish line April 15, 2013. The boy’s younger sister, Jane, 6, had a leg blown off. Their older brother, Henry, suffered minor injuries.

Their father, testifying at Tsarnaev’s federal death penalty trial, spoke in a slow, halting voice but remained largely composed as he described the chaos and confusion.

He said he scooped Jane up in one arm and took Henry in the other and “tried to shield both of their eyes” from the carnage around them as he took them away.

Richard took the stand as federal prosecutors continued trying to drive home the consequences of the attack in such heartbreaking detail that Tsarnaev’s lawyers objected — and were overruled.

Tsarnaev, 21, showed no reaction to the testimony.

Some of the women on the jury appeared to wince at times during Richard’s testimony. Spectators in the courtroom could be heard crying quietly, including Rebekah Gregory, who lost a leg in the bombing.

Richard said he himself suffered shrapnel injuries, burns on his legs and two perforated eardrums. His wife, Denise, was blinded in one eye and had other injuries.

Defense lawyer Judy Clarke has admitted that the former college student took part in the bombings. But in a bid to save Tsarnaev from a death sentence, she argued that he was influenced by his older brother, Tamerlan, who was killed in a getaway attempt days after the bombing.

Also Thursday, Jeff Bauman — who lost both legs in the attack and was photographed being taken away that day in one of the most widely seen images of the tragedy — testified that he locked eyes with one of the bombers shortly before the blasts.

“He was alone. He wasn’t watching the race,” said Bauman, who walked slowly into court on two prosthetic legs. “I looked at him, and he just kind of looked down at me. I just thought it was odd.”

Later, from his hospital bed, Bauman remembered the man’s face clearly enough to give the FBI a description of someone authorities say turned out to be Tamerlan Tsarnaev.